1. Field of the Invention
The present invention relates to a belt type continuously variable transmission, and more particularly to an anti-rotation structure for a balance chamber snap ring of a belt type continuously variable transmission with which axial direction movement of a back surface member forming a back surface of the balance chamber can be prevented.
2. Description of the Related Art
In a belt type continuously variable transmission, a primary pulley and a secondary pulley sandwiching a belt are both constituted by a fixed sheave formed integrally with a pulley shaft and a movable sheave that is free to move in a length direction of the pulley shaft. Thrust is applied to the movable sheave by supplying a required oil pressure to an oil pressure chamber provided on a back surface of the movable sheave, and as a result, the movable sheave sandwiches the belt in cooperation with the fixed sheave such that power transmission is performed in accordance with a frictional force generated at that time. Since the oil pressure chamber is formed on the back surface of the rotating movable sheave, a centrifugal oil pressure is generated in oil (operating oil) contained in the oil pressure chamber. When a sheave thrust generated by the centrifugal oil pressure exceeds a thrust required to ensure that the belt does not slip, belt friction increases unnecessarily.
Therefore, in a conventional belt type continuously variable transmission, a balance chamber (balance oil chamber) is provided on the back surface of the movable sheave oil pressure chamber in order to balance the centrifugal oil pressure (see Japanese Patent Application Publication No. 2006-275154 (paragraph 0016, FIG. 4)).
FIGS. 4A and 4B of Japanese Patent Application Publication No. 2006-275154 show a condition in which axial direction movement of a cover member 27a serving as a back surface member that forms a back surface of a balance chamber (balance oil chamber) 29a is prevented by a balance chamber snap ring that is engaged with a ring groove provided on an inner side of an outer peripheral portion (cylinder 27) of a movable sheave 21b. 
Typically, the cover member 27a is held anti-rotationally on the movable sheave 21b by engaging a latch portion (not shown) that projects from an outer peripheral portion of the cover member 27a in a radial direction of a pulley shaft with a recessed portion provided on the inner side of the outer peripheral portion (cylinder 27) of the movable sheave 21b. The balance chamber snap ring (to be referred to hereafter as a snap ring), on the other hand, is not provided with a latch portion such as that provided on the cover member 27a. Instead, the snap ring used for the balance chamber is formed in a C shape, and is fitted into a ring groove under the application of an external force that makes an outer diameter thereof smaller than an inner diameter of the ring groove. Accordingly, the snap ring is pressure-fitted to the ring groove by an elastic force that returns the outer diameter of the snap ring to its original state when the external force is removed.
Incidentally, in the belt type continuously variable transmission described above, the movable sheave presses the belt from both sides while rotating, and therefore external forces are applied to the movable sheave from various directions. Hence, the external forces applied to the movable sheave may cause the snap ring fitted into the ring groove of the movable sheave to deform elastically in a direction of making the outer diameter thereof smaller than the ring groove.
When the snap ring deforms elastically within the ring groove in the direction of making the outer diameter thereof smaller than the ring groove, a pressure fitting force decreases, causing the snap ring to rotate independently about the pulley shaft, and as a result, mutual joining surfaces of the snap ring and the cover member become worn.